As used herein, ACFM is a gas flow rate meaning actual cubic feet per minute, as opposed to standard cubic feet per minute. Surge point refers to a condition where a stage goes into surge. Approach to surge, as it implies, means closeness to surge. Imbalance of stage flows relative to surge means that the stages are at different approaches to surge. Design flow means the design flow rate to the inlet of the first stage and percentage of design flow means the percentage of design flow when that is considered as 100 percent. A selected percentage of design flow adequate to protect a stage from surge, means a flow rate (to the inlet of the first stage) which is considered to safely protect the stage in question from surge. A critical stage is any one which is closer to surge than one which is thus adequately protected. A limiting stage is the stage which is at the closest approach to surge and in conventional practice is the one that sets the rate of recycle.
When controlling surge by recycle according to the conventional practice, a description of which may be found in U.S. Pat. No. 4,230,437, a portion of the high pressure discharge gas stream is expanded and then recycled to the inlet of the first stage to provide for more volume. There is an energy penalty in the expansion and re-compression of this recycle stream. Furthermore, the various stages of the compressor will usually be at different approaches to surge but the recycle rate is set by the limiting stage, i.e., which is at the closest approach to surge and the other stages that may not need as much recycle to be protected receive unnecessary recycle as it passes through those other stage and has to be re-compressed. It follows that not only the necessary but the unnecessary portion of recycle has to be re-compressed, which wastes energy.
The percentage of design flow rate (to the first stage of the compressor) at which a stage goes into surge may differ for the various stages, simply because of the original design of the compressor and/or changes in operating conditions such as pressure. Stated differently, at a given percentage of design flow rate, the stages may each be at a different approach to surge. However, the approach to surge may be affected in another way. For example, if a particular stage other than the first stage receives an import of additional feed from another source, it will be put a further distance away from surge since it is receiving more volume than the percentage of design flow rate to the first stage. Thus, approach to surge may be a result of built in effects but it also may be manipulated, the term being used herein regardless of how achieved.